Prep the slow cooker: Add the sliced onion and minced garlic to the bottom of the crockpot.
Season the chicken: Pat the chicken dry. Sprinkle both sides with the taco seasoning, plus a pinch of salt and pepper.
Layer and add liquids: Place the chicken on top of the onions. Pour in the salsa and chicken broth.
If using, add tomato sauce and chipotle.
Cook low and slow: Cover and cook on Low for 4–6 hours or High for 2.5–3.5 hours. Chicken should be tender and easily shredded with two forks.
Shred the chicken: Transfer chicken to a plate and shred with forks. Return it to the crockpot and stir to coat in the sauce.
Finish with lime: Stir in lime juice and taste.
Adjust seasoning with more salt, pepper, or taco seasoning as needed.
Warm the tortillas: Heat tortillas in a dry skillet or over a low flame until soft and pliable.
Assemble tacos: Add a generous scoop of shredded chicken to each tortilla. Top with cilantro, onion, avocado, queso fresco, and a squeeze of lime.
Serve: Pair with rice, beans, or a crisp slaw if you want to round out the meal.